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When shopping for a solar electric system for your home or business, make sure that the dealer that you are speaking with is quoting you in actual PTC rated kilowatt hours per day.
We have noticed several dealers that are listing only STC rated kilowatt hours per day which are not real world numbers. Make sure when comparison shopping that you are comparing apples to apples. Insist upon receiving an actual system PTC rating which includes the efficiency rating of both the modules and inverter.
STC in an acronym for "Factory Standard Test Conditions" which is 1,000 watts per square meter solar irradiance, 1.5 Air Mass and a 25 degrees C. cell temperature. PTC is an acronym for "PV USA Test Conditions" which were developed at the PV USA test site at the University of Davis, California. The PTC rating represents a more real life condition of 1,000 watts per square meter solar irradiance, 1.5 Air Mass, and 20 degrees C. ambient temperature at 10 meters above ground level and wind speed of 1 meter per second.
The ambient temperature rating is a better standard than factory conditions because silicon solar cells average about 20 degrees C. above ambient temperature in the real world, cell voltage drops as temperature increases. A module's power output in real life conditions is lower than the power measured at the factory where cell temperature is maintained at a controlled 77 degrees F. (25 C).
Cell voltage drops about 0.08 volts per degree C. in environments which exceed 25 degrees C. Thus, an STC rating of 17 volts can actually become a PTC rating of 15 or 16 volts. Using Ohm's Law, volts times amps is equal to watts which equals power, so a reduced voltage, means reduced watts.